Mapper: move map_song_detach() to db/DatabaseSong.cxx

Use Storage::MapUTF8() internally, don't use global variables.

src/DetachedSong.hxx

@@ -30,9 +30,12 @@
 #include <time.h>
 
 struct LightSong;
+class Storage;
 
 class DetachedSong {
 	friend DetachedSong map_song_detach(const LightSong &song);
+	friend DetachedSong DatabaseDetachSong(const Storage &db,
+					       const LightSong &song);
 
 	/**
 	 * An UTF-8-encoded URI referring to the song file.  This can